1. Field of the Invention
The present invention relates to a voltage-controlled oscillating circuit. More particularly, it relates to a low noise voltage-controlled oscillating circuit which removes the noise of a power source to improve low frequency noise characteristics.
2. Description of the Related Art
[Prior Art]
A conventional voltage-controlled oscillating circuit (the voltage-controlled oscillator: VCO) is a circuit which varies an oscillation frequency in accordance with a direct-current voltage applied to a control terminal.
Moreover, the conventional VCO has a constitution directly connected to an external power source sometimes, and the VCO having such a constitution is influenced by the power source noise of the external power source, especially by the noise having a frequency which is as low as several kHz or less as it is, so that phase noise characteristics in such a band deteriorate.
[Deterioration of Phase Noise Characteristics due to Power Source Noise: FIG. 4]
The deterioration of the phase noise characteristics due to the power source noise in the conventional VCO will be described with reference to FIG. 4. FIG. 4 is a diagram showing the deterioration of the phase noise characteristics due to the power source noise. In FIG. 4, the abscissa indicates a frequency (the offset frequency), and the ordinate indicates the phase noise.
Moreover, as shown in FIG. 4, as to the deterioration of the phase noise characteristics due to the power source noise in the conventional VCO, a lower curve indicates the characteristics which are not influenced by the power source noise, and an upper curve indicates the characteristics influenced by the power source noise. The characteristics influenced by the power source noise comparatively deteriorate.
[Use Method of VCO for decreasing Power Source Noise: FIG. 5]
To prevent the above deterioration of the phase noise characteristics due to the power source noise, it is considered that a ripple filter circuit, a large-capacity low equivalent series resistance (ESR) capacitor and the like are used in the power source as shown in FIG. 5.
FIG. 5 is a diagram showing a use method of the VCO in which the power source noise has been decreased.
In FIG. 5, a ripple filter circuit 200 is inserted into a power source terminal of a VCO 100, and the power source terminal is further connected to a low ESR capacitor.
The ripple filter circuit 200 comprises a transistor Tr including a collector connected to a power source and an emitter connected to a power source input terminal of the VCO 100, the collector is connected to a base via a resistor R, and the base is connected to one end of a capacitor C having the other end connected to ground.
[Prior Art]
It is to be noted that examples of a concerned prior art include Japanese Patent Application Laid-Open No. 08-107309 titled “Voltage-controlled Oscillating Circuit” (applicant: Toshiba Corp.) [Patent Document 1], Japanese Patent Application Laid-Open No. 09-191215 titled “Voltage-controlled Oscillating Unit” (applicant: Mitsubishi Electric Corp.) [Patent Document 2], and Japanese Patent Application Laid-Open No. 11-225020 titled “VCO Circuit” (applicant: Murata Mfg. Co., Ltd.) [Patent Document 3].
Patent Document 1 discloses a voltage-controlled oscillating circuit in which a capacitor C10 is connected to a base of an amplifying transistor Tr2 of a buffer amplifier circuit section 1 to connect the base to ground in terms of a low frequency alternate current, and the buffer amplifier circuit section 1 operates as a ripple filter with respect to a low frequency noise applied to the section through a power source terminal B, and operates as a usual amplifier with respect to a signal applied from an oscillating circuit section 2.
Patent Document 2 discloses a voltage-controlled oscillating unit in which a collector of a transistor is connected to ground in terms of a high frequency, and a high pass filter is inserted as a phase advance circuit for canceling a phase lag, whereby an output frequency can be extracted as a low phase noise signal in the vicinity of a resonance frequency of an inductor resonator.
Patent Document 3 discloses a VCO circuit in which a resistor 4, an inductor 12, a resonator 13 and a capacitor 7 are connected in series between an input terminal 3 and a base of a transistor 8 as a resonant circuit, and a connecting point between the resistor 4 and the inductor 12 is connected to one end of a variable capacitor 6 having the other end connected to ground. An output of the resonant circuit is given to a tank circuit 2, and a parallel circuit constituted of a resistor 14, an inductor 15 and a resistor 16 is connected in series between an emitter of the transistor 8 and the ground.